A signal detection device of the present invention includes a first control unit configured to determine whether there are inputs of a plurality of external signals connected thereto, a second control unit configured to be activated after the first control unit is activated, a first input unit connected directly to the first control unit and configured to input a first external signal, which is one type of the plurality of external signals, and a second input unit connected to the first control unit via the second control unit and configured to input a second external signal, which is another type of the plurality of external signals, and the first control unit determines whether there is an input of the first external signal before the second control unit is activated and determines whether there is an input of the second external signal after the second control unit is activated.